The dual capacity compressor is a kind of reciprocating type compressor of which piston stroke as well as a compression capacity vary with an eccentric sleeve rotatably coupled to a crank pin of a crankshaft, a position of which with respect to the crank pin is dependent on a direction of rotation of the motor and the crankshaft. Since the dual capacity compressor has a compression capacity that can be varied with a required load, the dual compressor is used widely in apparatuses which require compression of working fluid, particularly in home appliances operative in a refrigeration cycle, such as a refrigerator, for enhancing an operation efficiency. A U.S. Pat. No. 4,236,874 discloses a general dual capacity compressor, referring to which a related art dual capacity compressor will be explained, briefly. FIG. 1 illustrates a section of a dual capacity compressor disclosed in U.S. Pat. No. 4,236,874, and FIG. 2 illustrates operation of the dual capacity compressor, schematically.
Referring to FIG. 1, the dual capacity compressor is provided with a piston 7 in a cylinder 8, a crankshaft 1, a crank pin 3 having a center 3a eccentric from a center of the crankshaft 1a, an eccentric ring 4 coupled on the crank pin 3, and a connecting rod 6 connected to the eccentric ring 4 and the piston 7, as key components. The eccentric ring and the connecting rod 6 are rotatable both with respect to the other, and the center 3a of the crank pin. There are release areas 9 in contact surfaces of the crank pin 3 and the eccentric ring 4, and a key 5 for coupling the crank pin 3 and the eccentric ring 4 together fitted in the release areas for coupling the crank pin 3 and the eccentric ring 4. The operation of the dual capacity compressor with respect to the compression capacity will be explained.
Referring to FIG. 2, in the dual capacity compressor, a stroke of the piston 7 is regulated by an eccentricity varied with a position of the eccentric ring 4, and, if a large capacity is required, the crank shaft 1 is rotated in a clockwise direction (regular direction) and, if a small capacity is required, the crank shaft 1 is rotated in a counter clockwise direction (reverse direction). In detail, FIG. 2A illustrates a moment the piston 7 is at a top dead center during a clockwise rotation, and FIG. 2B illustrates a moment the piston 7 is at a bottom dead center during a clockwise rotation, when the stroke Lmax is the greatest because the eccentricity is the greatest. And, FIG. 2C illustrates a moment the piston 7 is at a bottom dead center during a counter clockwise rotation, and FIG. 2D illustrates a moment the piston 7 is at a top dead center during a counter clockwise rotation, when the stroke Lmin is the smallest because the eccentricity is the smallest.
However, during the foregoing operation, the crank pin 3 and the eccentric ring 4 are involved in centrifugal forces, respectively caused by their rotation around the center 1a of the crank shaft, exerting on a point between the shaft center 1a and the pin center 3a, and a point on a line extended from the shaft center 1a to the a center of gravity of the ring 4a, respectively. Therefore, different from FIGS. 2A and 2B, as lines of actions are not one the same line, a local rotating moment is occurred at the eccentric ring 4 as a product of a vertical distance ‘d’ to the pin 3 and its own centrifugal force, acting in a direction the same with a direction (counter clockwise direction) of rotation of the crank shaft 1. Since the crank pin 3 and the eccentric ring 4 are members that can make relative movement to each other, the rotating moment causes a relative rotation of the eccentric ring 4 in a direction of rotation of the crank shaft 1, releasing the key 5 from the crank pin 3 and the eccentric ring 4, and leaving the eccentric ring 4 and the key 5 to move together as shown in FIG. 3.
Moreover, as shown in FIG. 3, during operation, for an example, in the clockwise direction, a pressure ‘P’ (a pressure of re-expansion of the working fluid) in the cylinder after compression pushes the eccentric ring 4 to a direction of rotation of the crank shaft 1, to cause the eccentric ring 4 to make a relative rotation with respect to the crank pin 3 in a rotation direction of the crank shaft. At the end, such a relative rotation makes operation of the compressor unstable, and performance of the compressor poor.
In fact, the relative rotation is occurred because the key 5 cannot fix the crank pin 3 and the eccentric ring, perfectly. The key 5 rolls within the release area whenever the direction of rotation of the crank shaft is changed, to cause serious wear at respective contact surfaces, that shortens a lifetime of the compressor.
In the meantime, other than the U.S. Pat. No. 4,236,874, there are many patent publications that disclose technologies of the dual capacity compressors, which will be explained, briefly.
Similarly, U.S. Pat. No. 4,479,419 discloses a dual capacity compressor provided with a crank pin, eccentric cam and a key. The key is fixed to the eccentric cam, and moves along a track in a crank pin when a direction of rotation of the compressor is changed. However, since the key cannot fix the crank pin and the eccentric cam perfectly, the U.S. Pat. No. 4,479,419 also has unstable operation caused by the relative rotation.
U.S. Pat. No. 5,951,261 discloses a compressor having an eccentric part with a bore of a fixed inside diameter formed across the eccentric part, and an eccentric cam with another bore with a diameter the same with the eccentric part formed at one side thereof. A pin is provided to the bore in the eccentric part, and a compression spring is provided to the bore in the eccentric sleeve. Accordingly, when the bores are aligned during rotation, the pin moves to the bore in the cam by a centrifugal force, that couples the eccentric part and the eccentric cam, together. However, since the U.S. Pat. No. 5,951,261 is provided with only one bore in the eccentric cam, the U.S. Pat. No. 5,951,261 can couple the eccentric part and the eccentric cam together only when the compressor rotates in a particular direction. Moreover, an operation reliability cannot be secured, since an exact movement of the pin from the eccentric part to the cam through respective bores is difficult.